1. Technical Field of the Invention
This invention is related to a process for measuring the friction and determining the coefficient of friction of sheet-shaped materials, especially paper and similar materials, especially based on cellulose, e.g. from wood, such as softwood or hardwood, but also paper from other kinds of cellulose materials and also paper from other materials as well as other sheet-shaped materials of organic or inorganic materials, also including metals or similar materials, as well as plastics, elastomers, fabrics, such as woven or non-woven fabrics, etc.
The expressions "sheet", "sheet-shaped" and "layer" herein is on the first hand intended to comprise a body having an extension in one direction which is small, preferably at most 1:5, especially at most 1:10, in relation to the extension thereof in two thereto and in relation to each other perpendicular directions.
According to this invention it is possible to determine the friction of a sheet-shaped test material, such as a sheet, against another sheet-shaped material or the same kind of sheet-shaped material, but also against a material with another shape than the sheet-shape, e.g. a piece of metal, plastics, elastomeric material, etc., having a suitable shape.
In the following the surface of the test material having sheet-shape is usually only called "test surface" and the surface of the material against which the friction is measured is called "counter-surface" but alternatively the material of a friction surface forming body which does not exhibit "sheet-shape" can be the material for which the friction against various other materials is to be determined.
2. Prior Art
A common way of determining the friction against a sheet-shaped material is according to the level of the prior art to arrange the sheet-shaped material on a flat surface acting as a support for the sheet-shaped material, and measuring the force which is required for bringing a counter-surface forming body having a known weight to move on the surface of the sheet-shaped material. The transverse or force which said body exerts against the surface (especially the normal force, i.e. a force acting in a direction normal or perpendicular to said surface) depends on the weight of the body under the influence of the gravity force. The sheet-shaped material can be arranged on a supporting surface, the plane of which can be tilted in a degree which is required for bringing the counter-surface forming body to start moving on said surface by the influence of the gravity force component acting in the direction of the plane of said surface, whereby the slope of said plane against the horizontal plane when the body starts sliding gives a measure of the force of friction and coefficient of friction in question, or else the body can be pulled on the preferably horizontally arranged surface of the sheet-shaped material and the pulling force and normal force and preferably also the surface load are measured when the body starts to slide. Said previously known methods have, however, substantial disadvantages caused e.g. by uneven load of the force of friction, the appearance of jumping movement of the body, the creation of acceleration forces which disturb the measurement of the force of friction, that the application of the normal force may take place under a sidewise movement of the normal force and that the application of the normal force may take place with jumps or with variations from one experiment to the other, and the measuring methods are often time-consuming and complicated, etc.
It has now surprisingly been revealed that it is possible to completely or to an extential degree eliminate said deficiencies and also to simplify the measurement with the process and the device according to this invention.